After their synthesis, secretory proteins translocate into the endoplasmic reticulum (ER) where they are post-translationally modified and properly folded. To reach their native conformation, many secretory proteins require the formation of intra- or inter-molecular disulfide bonds (1). This process is called oxidative protein folding. Disulfide isomerase (PDI) catalyzes the formation and isomerization of these disulfide bonds (2). Studies on mechanisms of oxidative folding suggest that molecular oxygen oxidizes the ER-protein Ero1, which in turn oxidizes PDI through disulfide exchange (3). This event is then followed by PDI-catalyzed disulfide bond formation on folding proteins (3). Other ER resident proteins that possess the thioredoxin homology domains, including endoplasmic reticulum stress proteins 72, 57 and 44 (ERp72, ERp57 and ERp44), constitute the PDI family (4,5,6). The ER also contains a pool of molecular chaperones, including Grp94, to help proteins fold properly. Grp94 is a glucose-regulated protein (7) with sequence homology to Hsp90 (8). BiP is another chaperone whose synthesis is increased when protein folding is disturbed. BiP binds to misfolded proteins to prevent them from forming aggregates and assists in proper refolding (9).